The mean or expected value of an exponentially distributed random variable X with rate parameter λ is given by E ⁡ [ X ] = 1 λ . {\displaystyle \operatorname {E} [X]={\frac {1}{\lambda }}.} In light of the examples given below , this makes sense: if you receive phone calls at an average rate of 2 per hour, then you can expect to wait half an hour for every call.